As the Nutrition Unit, we would be participating in the study of a low-fat diet (20% of calories as fat) on development of breast cancer in a high-risk group of post-menopausal women. Our responsibilities include design of an appropriate dietary regimen to achieve the goal of 20% of calories as fat; training the nutritionists who work in the Clinical Units; identifying noncompliance to the diet in the Experimental group ("drop-outs") and change of diet in the Control group ("drop-ins"); and providing appropriate dietary information on individual patients to the Clinical Units and on nutrient values and dietary fat levels to the Statistical Unit. We will use a rapid semiquantitative food frequency questionnaire (SFFQ) to determine eligibility for the study and as a check on dietary adherence in the Experimental Group, every 2 months for the initial 18 mos, and every 6 months thereafter. For more quantitative nutrient data, a 3-day diet record (3DDR) will be administered to all subjects in the Experimental Group at entry and annually thereafter. The 3DDR will be analyzed by a program developed here, using a Nutrient Data Bank from the Univ. of Mass. The Control Group will be followed by the Clinical Unit with a nonintervention policy by the Nutrition Unit. We will obtain a short SFFQ annually from each subject, and a 3DDR on a 10% sample of the Control Group each year. These data are felt to be needed in order to identify "drop-ins". As another check on dietary adherence, and a means by which to obtain biochemical parameters for diet and clinical outcome, we propose to obtain blood from all subjects, for measuring serum lipids (total cholesterol, triglycerides and HDL and LDL cholesterol). Based on previous studies, the low-fat diet should produce lower serum cholesterol and shifts in LDL and HDL cholesterol. Ten percent of the Control and Experimental Group will be sampled yearly to obtain blood for determinations of estrogens (estrone, estradiol, and free estradiol) and apolipoproteins (A-I and B). The objectives of our proposal are to provide advice, training and written materials to carry out the dietary mandate of 20% of calories as fat in each Clinical Unit. We also wish to obtain accurate analyses of nutrients, using both a food frequency questionnaire and a 3DDR, to permit sound statistical correlations. The biochemical measurements will provide an additional measure of dietary adherence and will also allow a comparison to be made of serum lipids and estrogen levels with dietary parameters and clinical occurence of breast cancer.